1. Field of the Invention
The present invention relates to a two-component developing method regulating a two-component developer on a developer bearer with a regulation member, i.e., a doctor blade, and transferring the two-component developer after regulation onto a latent image on an image bearer to develop the latent image.
2. Description of the Background Art
Typically, in electrophotographic image forming apparatuses such as copiers, printers, and facsimiles, an electrostatic latent image is formed on an image bearer such as photoreceptor drums and belts according to image information and an image developer develops the electrostatic latent image to form a visual image.
In an image developer using a magnetic brush developing method with a two-component developer, the two-component developer forms brush-chain-shaped ears on a developer bearer, and a toner in the developer is fed onto the latent image on the image bearer in a developing area where the magnetic brush contacts the image bearer.
The developer bearer is typically formed of a cylindrically-shaped sleeve (developing sleeve), and includes a magnetic material (magnetic roller) forming a magnetic field so as to form ears of developer on the surface thereof. Ears of carrier are formed on the sleeve along magnetic lines of force generated by the magnetic roller, and charged toner adheres to the carrier. The magnetic roller has multiple magnetic poles, with stick-like magnets forming the respective magnetic poles. In particular, the magnetic roller has a main magnetic pole used for developing, which attracts developer at a developing area on the surface of the sleeve and causes the developer to stand up on end like ears of grain. When either the sleeve or the magnetic roller moves, ears of developer on the surface of the sleeve move. The regulation member, i.e., the doctor blade, contacts the ears of a developer to give them an even height and an even quantity and transfers them and to the image bearer.
Developer transferred to the developing area forms ears along magnetic lines of force generated by the developing main magnetic pole, and the resultant chain ears of the developer flexibly contacts the surface of the latent image bearer. The chain ears of the developer feeds toner to (that is, develops) an electrostatic latent image in friction therewith, based on a relative linear speed difference with the latent image bearer.
Toner fusing to the doctor blade occasionally causes imaging problems.
Toner or foreign particles adhering to melted toner adheres to the doctor blade and changes the size of a gap between the developer bearer and the doctor blade, and the amount of the developer is not uniformly regulated, resulting in imaging problems such as background fouling (foggy images) and striped images. The exact diameter of the toner largely determines whether and to what extent the toner fuses to the doctor blade. Toner that includes a large amount of fine powdery toner is more likely to cause the toner to fuse to the doctor blade. Fine powdery toner is, e.g., toner having a small particle diameter; i.e., not greater than 2 μm.
The precise mechanics of fusing are as follows. Fine powdery toner is likely to enter concavities of the doctor blade, and the toner remaining on the surface thereof, being vulnerable to a heat, is melted by the heat of the doctor blade. Toner as well as paper powder further adhere to the toner melted on the surface of the doctor blade, resulting in build-up of a large amount of extraneous matter.
The amount of fine powdery toner having a small particle diameter which is included it he toner as a whole can be decreased to prevent the toner fusion bond to the doctor blade. However, toner having a small particle diameter is advantageously used to produce quality images, and consumes less heat energy when fixed. In short, toner having a small particle diameter is advantageous in that it can be used to produce quality images and save energy, but which tends to anchor to the doctor blade.
There are suggested methods of reversing the developer bearer, etc., to prevent toner fusing to the doctor blade and adherence of foreign particles thereto.
Typically, as Japanese published unexamined application No. 2005-274756 discloses, toner fusion to the doctor blade often occurs in one-component developing methods, in which the doctor blade regulates a layer thickness of a developer while pressed against a developing roller. In this case, the type of material used for the part of the doctor blade that contacts the developing sleeve and its surface roughness are controlled to prevent toner fusion.
Japanese Patent No. 3696404 discloses a method of regulating a surface roughness of the end of the regulation member facing an image bearer so as to smoothly regulate the layer thickness of a two-component developer with a sufficient rectification effect on the fluidity of the developer. However, the purpose and effect of this approach is strictly to improve rectificability of the developer, and thus the object thereof is different from that of the present invention.
Japanese Patent No. 2981802 discloses a toner including particles having a particle diameter not greater than 4.0 μm in an amount of from 20 to 40% by number, and particles having a particle diameter of from 4.0 to 8.0 μm in an amount not less than 60% by number. However, the content of particles having a particle diameter not greater than 2.0 μm is not disclosed.
Similarly, Japanese Patent No. 3168366 discloses a toner including particles having a particle diameter not greater than 4.0 μm in an amount of from 10 to 70% by number, and particles having a particle diameter not greater than 5.0 μm in an amount not less than 40% by number. However, here too the content of particles having a particle diameter not greater than 2.0 μm is not disclosed.
Japanese Patent No. 3351347 discloses a toner including particles having a volume-average particle diameter of from 2.0 to 5.0 μm and a particle diameter not greater than 5.0 μm in an amount not less than 90% by number, and particles having a particle diameter not greater than 1.0 μm in an amount not greater than 20% by number. However, again, the content of particles having a particle diameter not greater than 2.0 μm is not disclosed.
For these reasons, a need exists for a two-component developing method, an image developer, and a developer capable of producing high-quality images, having low-temperature fixability, and preventing the toner from fusing to the doctor blade.